What lens for using a M3 in Astrophotography?

Roger Bloemers wrote:

The question is in the title. I know a little girl with a M3 Canon and is interested in astrophotography. What lens might I suggest to her. I do not use the M series photography. (yet) thanks so much

EOS M6 + EF-M 22mm f/2 STM lens
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ASTROPHOTOGRAPHY with the EOSM
Any lens can be used for Astrophotography.  And any EOSM camera can do the job. The M3 is quite capable and has some benefits over the original EOSM camera.  You don't need a fast lens to shoot the details of the moon but you will want a fair bit of zoom in order to see lunar details like craters and mountain shadows etc.  For pictures of the Milky Way, you'll want the opposite.  For this you'll want a wider lens that is also fast.  The EOSM is a very capable camera for Astrophotography but as you noted, it's for a little girl.  Hence, one of the best lenses (see pic above) is also the most ideal.
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MILKY WAY shots
[wide, fast lenses.  Preferably f/2.8 or faster ...and wider than 35mm]
Any "fast" lens is ideal for the Milky Way.  Lenses that are f/4 or slower will be problematic and aren't bright enough, even if they are sharp enough, to capture images of the Milky Way without loosing detail from High ISO and long aperture exposures.
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LUNAR (Moon) shots
[Moon requires 150mm or greater]
If you want to get shots of the Moon or a Comet that is in the distance and quite small, then you'll want something with more zoom.  I've found that on the EOS-M, a zoom lens of at LEAST 135mm will show some craters and basic lunar details. You can just barely make them out with 100mm but around 300mm or more would be preferred.  This focal length will also capture comets that are too far to see with the naked eye.  But in order to capture greater lunar detail, you'll need to look at around 150mm or preferably much more.  Remember that the APS-C sensor on the EOSM cameras will multiply (magnify) the perceived Focal Length of any lens by 1.61x
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Another alternative is to obtain a telescope that allows for the fitting of a camera to the eyepiece. Small and relatively inexpensive spotting scopes (such as the Celestron Mak C90 series) will allow you to fit a camera either to the eyepiece (with a special clamp) or to replace the eyepiece with the camera body itself.
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COMETS etc
[Fast lenses (if possible) with a stronger zoom]
Ideally, you'd use an EQ mount with tracking for comets... but you can do it with a 100mm lens if it's f/2.8 or brighter on just a normal tripod.  I've had some success with fairly boring comets using the EF 135mm f/2.0L USM lens.  Again, if the comet is visible in the late afternoon sun you usually don't need a wide or fast lens to capture it.  But you can use almost anything you like if the comet is visible to the eye. 
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DEEP SPACE objects
[lens requirements vary, EQ mount with tracking is recommended]
With deep space objects where the subject is outside of our Solar System, a variety of lenses can be used on the EOSM cameras.  You can capture the Dwarf Galaxies that surround our own (the large and small 'Magellanic Clouds') with a 50mm lens that is f/1.8 or you can use the 22mm f/2 lens to do the same on a wider scale.  For subjects such as the Orion Nebula, even a 135mm zoom can be used.  You can see my first attempt below using 400mm but I really should have used a tripod for that shot.
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The one lens that ticks lots of boxes for general night-sky shots is the relatively affordable EF-M 22mm f/2 STM lens.  It's bright and quite wide enough for anything Milky Way related plus you can use it to stitch images together to make a sort of panorama if you want an even wider view.  You can't capture close shots of the moon with it but it's bright enough (fast enough) to capture very nice details in the Milky Way.
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The following images were all captured with my EOSM and EOSM6 camera using various lenses...  Note that I often use a Tungsten White Balance for my night shots.  This is just a personal preference.  These are single exposures with just a tripod for stability.
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EOSM + EF 24mm f/1.4L USM lens - Andromeda Galaxy - single exposure

EOSM + EF 135mm f/2 USM lens  - Comet Lovejoy

EOSM + EF-M 11-22mm f/4 IS STM lens. - note the muddy details in the Milky Way here.

EOSM + 24mm f/1.4L USM lens - two image panorama.  Note Milky Way (left) and dwarf galaxies on the right (Large and Small Magellanic Clouds)

EOSM + EF 50mm f/1.2L USM lens - Comet Lovejoy - with the tail just barely visible.

EOSM + EF 50mm f/1.2L USM lens - Large Magellanic Cloud (Dwarf Galaxy)

EOSM + EF 100-400mm f/4.5-5.6L IS USM II lens - handheld - 4 seconds *(note star trails)

EOS M6 + EF-M 22mm f/2 STM lens (single exposure + edited to enhance details)

EOSM + EF 100-400mm f/4.5-5.6L IS USM II  (uncropped)

EOSM + Celestron C90 Maksutov spotting scope via Clamp

EOSM + EF-M 22mm f/2 STM lens